Reiki is a Japanese holistic healing method for body and mind, and it can be very effective on a physical, emotional and spiritual levels.
There is a fairly large number of scientific publications on Reiki effectiveness; however, due to limitations of currently published studies, Reiki has not yet been scientifically proven to be effective. That does not mean that it is not effective, it simply means that there is not enough scientific evidence available today to demonstrate its effectiveness.
As a hospice doctor, I have observed Reiki in action at an inpatient hospice unit for over a decade, and was so impressed with how helpful it was to my patients (way beyond what one would expect from a placebo effect), that I have learned Reiki myself and now personally utilizing it.
One day, I am sure, scientific research will catch up, and the effectiveness of Reiki will not only be clear in practice, but also from a scientific point of view.